This invention relates to thin film heterojunction photovoltaic cells and methods of making them, and more particularly to the deposition of a film of a near intrinsic or n-type Class II B and tellurium containing semiconductor compound such as cadmium telluride and then heat treating said film to convert it to a suitably low resistivity p-type semiconductor compound with such film either being initially in contact with an n-type semiconductor substrate different from the film semiconductor compound or later having deposited thereon a layer of an n-type semiconductor compound different from the film semiconductor compound.
In general, the heat treating of thin film semiconductors is well known and practiced for a variety of reasons. For example, in the UK patent application No. GB2,006,268A, filed Oct. 11, 1978 by the University of Queensland, heat treatment of an electrodeposited film for one and a half hours in an atmosphere of argon at 350.degree. C. may be used to improve the crystallinity of the film. Similarily, in the U.S. Pat. No. 4,260,427, issued Apr. 7, 1981 to Ametek, Inc., heat treating in a nitrogen atmosphere at an elevated temperature on the order of 250.degree. C. for 60 minutes increases significantly the effficiency of a Schottky barrier solar cell, presumably by increasing the small crystal grain size. Alternatively, the Ametek patent teaches that such annealing treatment can be done in an oxidizing atmosphere to grow an oxide layer at 300.degree. C. and 60 minutes. Such latter approach was suggested in U.S. Pat. No. 4,035,197, issued July 12, 1977 to Eastman Kodak. In the Eastman Kodak patent, the surface of a layer of cadmium telluride is heated in an oxygen containing atmosphere for between about one minute and about twenty minutes at a temperature between about 250.degree. C. and about 500.degree. C. to alter the surface. A Schottky barrier solar cell was then formed by applying a metal electrode to such altered surface. Still another heat treating step is taught by U.S. Pat. No. 4,231,808, issued Nov. 4, 1980 to Fuji Photofilm, wherein an n-type cadmium telluride film is heated at a temperature between about 80.degree. C. and 200.degree. C. for about twenty to one hundred eighty minutes, i.e., rather gentle heating conditions, to form p-type cadmium telluride at the surface and thereby form a shallow p-n homojunction. Such conversion of the surface of n-type CdTe to p-type using low temperature heat treatment was earlier described by Yu A. Bodakov, et. al., in Fizika Tuerdogo Tela, Vol. 2, No. 1, pp-55-61, January, 1960. The Fuji patent expressly teaches that heat treatment under more severe conditions deteriorates the performance of the photovoltaic cell, probably by converting the entire thickness of the originally n-type cadmium telluride thin film to p-type.
From this review of the prior art, it is apparent that there has been a failure to appreciate the full potential use of heat treatment in the preparation of higher efficiency thin film photovoltaic cells. Consequently, an object of the present invention is an improved method of fabricating a heterojunction thin film photovoltaic cell utilizing a heat treatment.
Still another object of the present invention is a method of fabricating a thin film heterojunction photovoltaic cell which involves the conversion of a near intrinsic or n-type semiconductor film to p-type to form a p-n junction with a different n-type semiconductor layer.